Having completed 4 channels of Aleph 5 with conventional heatsinks, I recently acquired a forced air-flow heat sink for free (junk yard). As using a fan is no option (noise), my plan is to cut this heatsink into six "slices" and mount them in a 2 x 3 array next to each other. The fins will be vertically and the transistors will attach to the sides of the array. This array will be mounted in the middle of the amplifiers case, and air can flow from the bottom through the heatsink to the top of the case. Of course, no other device will be placed on top of the amplifier and the feet it stands on will be higher than usual to assure air convection.

The heatsink I have is 26 cm long (there will be 6 slices of 4 cm), 6 cm high and there are forty fins. The picture shows a similar heatsink.

Each section (40 fins * 4cm * 6cm) has a surface area of about 960 square cm, adding up to a total of 5760 square cm.

(1 inch = 2.54 cm)

Here is my question: Can anyone estimate what approximate K/W rating the resulting heat sink will have, taking into account the case which will force the air to flow through it? Will it be enough for two channels of Aleph 5?

You should definitely not use that sink without a fan. The guess of about 1 deg C/W doesn't sound too bad. You might have a look at the Fischer Elektronik Website (http://www.fischerelektronik.de/) for some data.